The VIAP may pose unacceptable risks like immediate threats to safety (e.g., fire or explosion potential from vapors or methane) or adverse health effects from inhalation of hazardous substances that represent short-term or acute (i.e., less than chronic) exposure concerns. The potential for these to occur is greatest within the lateral inclusion zone (LIZ). The evaluation is described below.
Fire and Explosion
Methane is a fire and explosion hazard that is commonly associated with the anaerobic biodegradation of organic material or petroleum. However, when methane migrates in the vadose zone via diffusion under the standard CSM, it will aerobically degrade rapidly like petroleum. The presence of methane without pressure may not necessarily indicate a risk from fire and explosion. When methane is present, an evaluation of the pressure differential should be completed as part of the analysis to determine the potential need for immediate interim response actions. If concentrations of methane that exceed 25% of the lower explosive limit (LEL) (1.25% by volume) are identified near a building, additional monitoring and assessment of the potential of fire and explosion should begin to ensure no unacceptable risk is occurring now or will occur in the future. ASTM E2993-23 Standard Guide for Evaluating Potential Hazard in Buildings as a Result of Methane in the Vadose Zone or EGLEs Guidance for Development on or near Methane Generating Sources provides additional information on Methane.
The assessment from fire and explosion is done using an appropriately calibrated field meter (e.g., four gas meter) until either the vapor source has been addressed through a response action, or there is sufficient information collected that affirms that the release does not pose a potential fire or explosion risk. Soil gas data may also be used. The monitoring frequency of the explosive conditions should be established on a case-by-case basis and be based on the site conditions present and the potential that concentrations exceed the LEL.
LELs for hazardous substances are available in R 299.50 Toxicological and Chemical-Physical Properties under Part 201. When concentrations of methane are detected in indoor air at a site, constant monitoring and ongoing assessment of the potential of fire and explosion should begin. The installation of calibrated gas leak detectors and alarms may be appropriate in situations that require frequent monitoring and can reduce the need to complete constant physical monitoring. If the concentrations ever exceed 10% of the LEL, evacuation and other immediate response actions should begin immediately while the methane is being mitigated.
There may be a potential risk from fire and explosion associated with petroleum immediately after the release has occurred and when NAPL has been found to be in contact or has entered a building. More information on this scenario can be found in Volume 4.
Short Term and Acute Health Effects
The screening levels and criteria for most of the hazardous substances associated with releases are based on 30-year (chronic) exposures. In 2016, the Toxics Steering Group (TSG) VIAP Workgroup was tasked by EGLE and the Michigan Department of Health and Human Services (MDHHS) to evaluate and develop indoor air screening levels for volatile substances that are protective against human health effects that may result from ongoing VIAP exposures and was revised in 2020. Several hazardous substances were identified by the TSG as having short term (acute) adverse health effects, with a typical exposure period assumed to be 24-hours. Additional information is available in Volume 6 – Volatilization to the Indoor Air Criteria.
Short term or acute risks are possible and need to be evaluated under the following conditions:
- Sub-slab soil vapor has been identified above site-specific volatilization to the indoor air criteria (SSVIAC) or screening levels for a chemical with identified acute health effects,
- Odors or chemical concentrations above acceptable indoor values have been identified within a building, or
- A vapor source has entered a building or an underground conduit that can transport vapor directly to a building.
If any of these situations have been identified, it is critical to immediately assess the risks and identify if there are response actions that must be immediately implemented to abate the unacceptable risks. The response actions should identify the vapor source causing the potential health effects, eliminate unacceptable levels of vapors from the building, and prevent vapors from continuing to infiltrate into the building.
Interim response actions should be completed consistent with the Volume 5 – Response Activity for the Volatilization to the Indoor Air Pathway (VIAP). Prior to implementing interim response actions representative sub-slab soil gas sampling is recommended and may also include indoor air sampling. If an evaluation is made in accordance with Rule 299.14(5) using more representative data such as soil gas, the soil gas data can be used to show compliance with the soil and/or groundwater VIAC. If indoor air samples are collected, they should be paired with a representative number of sub-slab samples or a conduit vapor sample (when appropriate). Indoor air samples should only be collected when the measured pressure differential between the indoor air and sub-slab indicates that advective flow of soil gas is from the subsurface to indoor air. Acceptable air values used to derive VIAC are provided in Volume 6 and may be used to determine if the risks from the VIAP are acceptable at the time of sampling. Although grab indoor air samples alone are not a reliable means to demonstrate a risk from the VIAP will not occur, they do provide a line of evidence that allows for an evaluation of the potential exposure at the time of sampling and/or the effectiveness of any implemented corrective or response actions. See Attachment C.3 for the number of sampling locations, number of sampling rounds, and sampling frequency.